So What Happened Tuesday Night?

Tuesday was an odd severe weather day. A screaming low level jet of up to 70 knots produced occasionally damaging synoptic winds throughout Connecticut including this impressive METAR from KBDR.

SPECI KBDR 190123Z 22033G41KT 2SM VCTS HZ BKN020 OVC032 24/21 A2971 RMK AO2 PK WND 22041/0123 LTG DSNT NW TSB21RAE17 PRESRR P0000

Not bad for a southerly LLJ case, right?

Courtesy: SPC

A deep trough and strong QG forcing lead to intense cyclogenesis across the eastern Great Lakes with a powerful surface cyclone tracking north into Quebec. While the strongest forcing bypassed southern New England to the north a rather impressive front swung through shortly after 00z. The warm sector was characterized by very modest CAPE with dew points approaching 70ºF.

00z 9/19/12 OKX sounding

While the observed instability was less than progged (models showed surface based CAPE values around 1000 j/kg around 00z in the warm sector) there was enough to maintain a line of low topped convection. While most of the convection was subsevere there was a small area in Connecticut that received a significant batch of wind damage as the line moved through. This occurred in a rural area of eastern Connecticut from Lebanon to Hampton, Chaplin, and Pomfret. Some of these towns were nearly 100% without power Wednesday morning.

Wednesday – 1 p.m. CL&P outage map. You can see sporadic pockets of wind related power outages from earlier synoptic wind gusts with a concentrated area of more significant damage from Lebanon to Pomfret associated with convective wind damage.

Looking back at the radar data it’s hard to discern what actually happened here. The wind damage occurred after the leading edge of the line moved through which is not what I would have expected.

There was an area of enhanced winds that radar detected behind the main line that tracked from Long Island Sound through Massachusetts. Here’s a look at 0.5º reflectivity and base velocity from OKX. You can see the main low level jet ahead of the narrow line of low topped convection. I also circled an area over East Haddam and Chester of note.

 

While it doesn’t look like much at 0258 UTC this small area of enhanced winds behind the squall line went on to produce damaging winds just to the northeast in Lebanon by 0315 UTC.

 

It appears the northern section of the squall line ran ahead of the LLJ as a pseudo-frontal wave developed on the LEWP. A separate convective element behind the squall line was able to tap into the still strong LLJ as the cell raced NE behind the leading edge of the squall line.

0.5º base velocity from OKX showed outbound velocities approaching 65 knots at 5000 ft AGL over Lebanon and Windham that continued to the north across Hampton, Chaplin, and Pomfret. Behind the squall line this convective element was apparently able to efficiently mix the tail end of the strong LLJ down to the ground with a widespread swath of wind damage.

While these squall lines in low CAPE/high shear environments rarely produce damaging winds in Connecticut this was a pseudo-exception. It was convection behind the squall line that lead to a period of damaging convective winds in eastern Connecticut.

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Nasty Storm to Start Meteorological Winter

A real nasty squall line across Pennsylvania and New York is moving northeast and will approach Connecticut by 3 p.m. or so.  In addition a strong southerly gradient is producing winds of 30-40 mph across the state.

Periods of heavy rain are likely through late this afternoon with wind gusts that could exceed 60 mph as the squall line and cold front move through. These squall lines aren’t like the ones you see in the summer – this time of year they’re also known as Narrow Cold Frontal Rain Bands. When these are strong enough winds from aloft can get mixed down to the surface and produce localized damage.

Behind this storm winter moves in with much colder temperatures in what looks like a much below average start to December.  Here’s today’s 12z GFS with 30s for highs in the forecast from Sunday straight on through next week.